1. Field of the Invention
This invention pertains to an objective lens system for a microscope. More specifically, this invention pertains to an objective lens system for a microscope equipped with a correcting ring that can correct for a change in aberration when the thickness of a cover glass changes. The system can also be used for fluorescence excited by UV rays.
2. Description of Related Art
When a biological microscope specimen is prepared for examination by a microscope, the specimen is usually placed on a glass slide and covered by a cover glass for sealing. A "coverglass-attached" specimen is formed in this way. The thickness and refractive index of the cover glass meet certain standards such as, for example, the Japanese Industrial Standards (JIS). In general, the thickness of the cover glass is set at 0.17 mm. This thickness is considered to be the standard thickness.
When the thickness of the cover glass is different from the standard thickness, the focusing ability of the objective lens system of the microscope is degraded. The influence of the thickness error is significant in that it results in the numerical aperture (often referred to as NA hereinafter) becoming large. When the NA increases to a certain degree, a portion of the objective lens system of the microscope is shifted to correct for the change in the aberration. Therefore, a correcting ring is usually provided in the objective lens system.
Certain techniques relating to objective lens systems for microscopes equipped with correcting rings are disclosed in Japanese Kokai Patent Application Nos. Sho 571982!-148717 and Sho 591984!-100409. Although the NA in these systems is not very large, corrections for changes in thicknesses of the cover glass on the order of a millimeter can be made. Other techniques, such as those disclosed in Japanese Kokai Patent Application Nos. Sho 611986!-275812 and Hei 51993!-119263, can be corrected differently. Although the correction ranges for the thicknesses of the cover glasses is small in these other systems, the various aberrations can be corrected to certain degrees over wide fields of view at high NAs.
In the biological field, the fluorescence microscope is now widely used. By fluorescence-dyeing a specific substance, cells can be observed without being damaged. A short-wavelength light beam is irradiated on the specimen (for excitation). The excited fluorescence is then observed by the fluorescence microscope. A typical example of short excitation wavelength light is light at the i-line (365 nm). Excitation light at a wavelength of 340 nm has also been used to observe calcium ions in living bodies.
In the systems disclosed by Japanese Kokai Patent Application Nos. Sho 571982!-148717 and Sho 591984!-100409, NAs are as small as 0.55 and 0.7, respectively. As a result, the resolving power of these systems is inadequate. When a correction for a change in the thickness of the cover glass is made in either of these systems, the coma is degraded. This is a disadvantage.
The objective lens system disclosed by Japanese Kokai Patent Application No. Sho 611986!-275812 has a NA which may be as large as 0.95. However, the coma is still degraded in this system by a change in the thickness of the cover glass.
The system disclosed in Japanese Kokai Patent Application No. Hei 51993!-119263 is designed for use without using fluorescent glass. It is difficult, therefore, to ensure a high light transmissivity at a wavelength of 340 nm for the lens configuration in this system. The high performance expected from a fluorescence microscope cannot be fully realized as a result. Because emitted fluorescence is always faint, fluorescence is generated by the excitation light in the glass used for forming the objective lens. This phenomenon is known as self-fluorescence. When this takes place, the contrast of the observed image deteriorates significantly.